The present invention relates to a main liquid fuel injection device for a single combustion chamber, having a premixing chamber, of a gas turbine with low emission of pollutants.
As is known, a gas turbine is a machine consisting of a compressor and a turbine with one or more stages, in which these components are interconnected by a rotating shaft and in which at least one combustion chamber is provided between the compressor and the turbine. In particular, reference is made here to the case in which a single combustion chamber is present.
Air from the external environment is supplied to the compressor where it is pressurized.
The pressurized air passes through a premixing chamber terminating in a nozzle or converging portion. At least one injector supplies fuel to this chamber, this fuel being mixed with the air to form a fuel-air mix for combustion.
The fuel required for the combustion is therefore introduced into the combustion chamber from a pressurized network, the combustion process being designed to cause an increase in the temperature and enthalpy of the gas.
A parallel fuel supply system, for generating a pilot flame, is also generally provided in order to improve the stability characteristics of the flame.
Finally, the gas at high temperature and high pressure passes through suitable ducts to reach the various stages of the turbine, which converts the enthalpy of the gas into mechanical energy which is available to a user.
It is well known that the primary considerations in the design of combustion chambers for gas turbines are the flame stability and the control of excess air, the aim being to establish ideal conditions for the combustion.
There is also a tendency to provide a mixture of air and fuel, by means of the premixing chamber, in order to achieve combustion with reduced emissions, mainly of nitrogen oxide and carbon monoxide. This is done by optimizing the excess combustion air factor.
More specifically, the prior art provides a premixing chamber immediately upstream from the combustion chamber.
Both the premixing chamber and the combustion chamber are surrounded by a cavity containing pressurized air circulating in the opposite direction to the flow of combustion products leaving the combustion chamber.
The aforesaid air (taken from the outlet of the axial compressor) is used as combustion air to be mixed with the fuel in the premixing chamber, and as cooling air for cooling the combustion chamber and the combustion products.
In order to achieve low emission of pollutants, especially nitrogen oxide, at all levels of loading of the turbine, in the system described above the passage of the combustion air from the cavity to the premixing chamber, through apertures in the outer surface of the latter, can be constricted.
The constriction is applied as a function of the quantity of fuel used, in such a way that the ratio between combustion air and fuel is kept constant at the optimal value.
To prevent the flame from being extinguished or becoming unstable in any way, a set of burners is provided with converging axes positioned circumferentially around the outlet of the premixing chamber, so that a corresponding set of additional flames is created in the combustion region.
These burners are supplied independently with additional fuel and with high-pressure air obtained by further compression of the air supplied by the turbine""s compressor; this air is sent to the burners through blades which are twisted so that an essentially helical motion is imparted to the air.
Thus, by using the additional flames of the burners, which are essentially pilot flames, not only is the main central combustion flame stabilized, preventing it from being extinguished, but, since the precise quantities of fuel and air used independently by the burners are known, the whole system can be regulated to achieve optimal and controlled ignition.
Furthermore, the quantity of additional fuel required for the burner flames becomes very low, and moreover it is entirely burnt in optimal conditions, so that the polluting emissions of nitrogen oxide are drastically reduced.
However, in order to reduce the emission of pollutants, it is essential that the liquid fuel injectors or main liquid fuel injection device provide a satisfactory distribution of the fuel-air mixture in the premixing chamber.
It is also necessary for the fuel supply channels to be kept clear, internally and externally, of carbon deposits which are formed as a result of the high temperature of the walls of the said channels.
It is therefore necessary to lower the temperature of the walls of the liquid supply channels, limiting their temperature to a maximum value: for example, General Electric usually specifies a maximum of 120xc2x0 C.
For this purpose, the liquid fuel injector is provided with internal passages for the cooling air, these passages surrounding all the liquid fuel supply channels. This air is then injected into different points of the air and fuel premixing channel.
The object of the present invention is therefore to overcome the drawbacks mentioned above, and in particular to provide a main liquid fuel injection device for a single combustion chamber, having a premixing chamber, of a gas turbine, which ensures a low emission of pollutants.
Another object of the present invention is to provide a main liquid fuel injection device for a single combustion chamber, having a premixing chamber, of a gas turbine with low emission of pollutants which also provides good flame stability and reduces the pressure oscillations in the combustion chamber.
Yet another object of the present invention is to provide a main liquid fuel injection device for a single combustion chamber, having a premixing chamber, of a gas turbine with low emission of pollutants which provides high combustion efficiency.
An additional object of the present invention is to provide a main liquid fuel injection device for a single combustion chamber, having a premixing chamber, of a gas turbine with low emission of pollutants which enables the average life of components subject to high temperatures to be increased, by reducing the possibility of formation of carbon deposits.
Another additional object of the present invention is to provide a main liquid fuel injection device for a single combustion chamber, having a premixing chamber, of a gas turbine with low emission of pollutants which is particularly reliable, simple, and functional, and has relatively low production and maintenance costs.
Advantageously, the main liquid fuel injection device for a single combustion chamber, having a premixing chamber, of a gas turbine with low emission of pollutants injects and atomizes the liquid fuel to be mixed with the air, thus creating a good distribution of fuel-air mixture before the inlet of the combustion chamber.
Furthermore, the main liquid fuel injection device for a single combustion chamber, having a premixing chamber, of a gas turbine with low emission of pollutants also provides self-cooling of the walls which are subjected to high temperatures, and also makes it possible to protect the outer surfaces and the liquid fuel injection channels of the device against the damage caused by the deposition of carbon residues.